refactor webapp model code that checks for submission completion

r? @mstriemer

**Refactoring**

- is_complete (returned a bool + list of reasons) **=>** details_complete (bool) + details_errors (list of errors)
- is_fully_complete (returned a bool + list of reasons) **=>** is_fully_complete (bool) + completion_errors (object of errors) + completion_error_msgs (list of errors)
- completion_errors, next_steps **=>** details_complete + content_ratings_complete + payments_complete

**Additions**

- tests

**Fixes:**

- fix toggling payment type to not set status to PENDING if app incomplete